1. Field of the Invention
The present invention generally relates to semiconductor device manufacture and, more particularly, to an apparatus and method for partial ion implantation.
2. Description of the Related Art
Ion implantation has various applications in the manufacture of semiconductor devices. Ion implantation generally is a process to change electric properties to a region of a semiconductor substrate or a wafer by implanting a dopant to the region. Such an ion implantation process is applied to, for example, a process of forming a well on the wafer, a process of forming various junctions, a process of adjusting a threshold voltage Vt in a channel region of a transistor, and the like.
When manufacturing semiconductor devices, the ion implantation process is generally performed such that dopants are implanted in a uniform dose on a whole area of the wafer. Thus, apparatus for ion implantation have been developed to create a uniform dose profile in the whole region of the wafer.
Semiconductor devices are gradually decreasing in size in response to design criteria. Thus, even if a uniform dose profile of the dopants is realized in the whole region of the wafer, characteristics of the device can differ at respective regions. That is, the device suffers from non-uniform device characteristics according to the respective regions.
For example, even if ion implantation for adjusting the threshold voltage Vt in the channel of the transistor is uniformly performed on the whole region of the wafer, actual distribution of threshold voltage Vt in the channel can be non-uniform according to the respective regions of the wafer. It is generally understood that such a non-uniform distribution is caused by a process margin or other process parameters for subsequent processes.
For example, if there is a variation in a device's critical dimension (CD) during gate patterning, causing a gate's critical dimension of the transistor to vary at the respective regions of the wafer, for example, a substantial length of the channel is changed. Such variation in the gate's CD is caused by the process margin or the other process variables during exposure or etching, which is performed upon gate patterning.
If there is a variation in the gate's CD, then effective threshold voltages in the channels of individual transistors will differ from each other. Due to such uneven distribution of the effective threshold voltage Vt, the respective transistors exhibit uneven operating characteristics over the whole region of the wafer.
A method has been developed to compensate for the uneven operating characteristics of the transistors dependant on the regions of the wafer. According to the method, ion implantation for adjusting the threshold voltage of the channel is performed to allow a dose of dopants to vary according to the regions of the wafer. For example, to compensate for the influence of the gate's CDs being different from each other at the respective regions on the threshold voltage Vt, ion implantation for adjusting the threshold voltage of the channel can be performed such that different doses of dopants are implanted to the respective regions of the wafer. That is, the uneven distribution of threshold voltage Vt of the channel can be compensated via ion implantation for adjusting a partially uneven threshold voltage Vt.
Because a general apparatus for ion implantation is designed to realize uniform dose distribution on the whole region of the wafer, it is substantially difficult to perform ion implantation to realize non-uniform dose distribution using such an apparatus. Thus, there is a need for an apparatus capable of partial, non-uniform ion implantation to provide a partially uneven dose distribution.